1. Field of the Invention
The present invention relates to a prism ballast type contact lens, a method of producing the same, and a mold assembly used in the method. More particularly, the invention is concerned with a method of producing such a contact lens in a simplified manner, a prism ballast type contact lens which is constructed to permit its production by that method, and a mold assembly used for producing the contact lens.
2. Discussion of the Related Art
As one example of contact lenses used for correction of astigmatism, or for correction of both of near and distant visions of the user, there is known a so-called "prism ballast type" contact lens which includes a front surface and a back surface, wherein the front surface has a center axis which is offset from that of the back surface defining a geometrical center axis of the lens, such that the contact lens has a thickness which continuously increases in a downward direction of the lens properly oriented in contact with the cornea of the user's eye. The prism ballast type contact lens further includes a slab-off area provided in at least a lower portion of the front surface.
FIGS. 1 and 2 show such a prism ballast type contact lens generally indicated at 2, wherein a back curved surface 4 is part-spherical or ellipsoidal following a profile of the cornea of the user's eye, and has a center axis A (FIG. 2) which is a geometric center axis of the contact lens 2, which is a center of a circle that defines the periphery of the lens, while a front curved surface 6 of the lens 2 is a toric or bifocal surface. In the contact lens 2, a center axis B (FIG. 2) of the front surface 6 is offset from the center axis A of the back surface 4 in the downward direction by a suitable distance, so that the lens 2 has a thickness which continuously increases in the downward direction of the lens 2 as placed on the eye with the desired rotational orientation. The contact lens 2 shaped to provide a ballast as described above is prevented from rotating on the cornea of the user's eye while the lens 2 is placed thereon.
In the contact lens of FIG. 2, at least a lower portion 8 of the front surface 6 is removed to reduce the thickness of the lens at that portion, and improve the wearing comfort of the lens as felt by the user, so that the contact lens 2 has a so-called "slab-off" area 10 having a part-spherical, conical or aspherical profile.
In general, the prism ballast type contact lens 2 as described above is produced mainly by: 1) a cutting operation wherein both of the opposite surfaces 4, 6 of the lens are formed by cutting, 2) a molding operation wherein both of the opposite surfaces 4, 6 of the lens are formed by molding, or 3) a combination of cutting and molding operations wherein one of the opposite surfaces 4, 6 of the lens is formed by cutting while the other surface is formed by molding. None of these methods, however, are satisfactory, since these methods require complicated and increased numbers of process steps in the manufacture of the lens, and largely depend on a high level of skill of the workers. Further, when both of the opposite surfaces of the lens are formed by the molding operation, it is necessary to prepare a large number of molds corresponding to respective specific different contact lenses, undesirably pushing up the cost of manufacture of the contact lens.
Described in detail, when both of the opposite surfaces 4, 6 of the prism ballast type contact lens 2 are produced by the cutting operation, the cut surfaces of the lens 2 are formed with dimensional accuracy higher than molded surfaces, i.e., the surfaces of the lens produced by molding, and exhibit excellent optical characteristics. This cutting method, however, requires complicated and cumbersome process steps, which include a step of forming the back surface 4 of the lens by using a precision lathe, a step of forming an optical portion of the front surface 6 of the lens, with the center axis of the front surface being offset by a suitable distance from the geometrical center axis of the lens, and a step of forming a slab-off area by cutting at least a lower portion of the front surface to reduce the thickness of the lens at that portion. This method not only requires cumbersome process steps of using the precision lathe, but also depends on the worker's skill to offset the center axis of the front surface from the geometrical center axis of the lens. This inevitably increases the burden of the worker, and causes an increase of a reject ratio, namely an increase in the number of defective products that must be rejected as being defective.
JP-A-57-98324 and U.S. Pat. No. 5,611,970 disclose examples of the method of producing the prism ballast type contact lens whose surfaces are formed by molding, using a mold assembly consisting of a male and a female mold half which cooperate to define a mold cavity having a profile following that of the intended prism ballast type contact lens. Described in detail, by polymerizing a suitable lens material filled in the mold cavity, the prism ballast type contact lens whose front and back surfaces have desired configurations is obtained. In this method, the mold cavity defined by the male and female mold halves should have a profile corresponding to that of the intended contact lens. Namely, a large number of various kinds of mold assemblies (male and female mold halves) need to be prepared depending upon the kinds of contact lenses having various configurations. Further, it is necessary to prepare and control an inventory of the large number of mold assemblies, inevitably increasing the cost of manufacture of the contact lens.
U.S. Pat. Nos. 4,113,224 and 5,110,278 disclose examples of the method of producing the prism ballast type contact lens, wherein one of the opposite surfaces of the lens is formed by cutting while the other surface is formed by molding. In this method, the intended prism ballast type contact lens is produced by first molding the back surface of the lens, then forming an optical portion of the front surface of the lens by cutting using the precision lathe, with the center axis of the front surface being offset by a suitable distance from the geometrical center axis of the lens, and finally forming a slab-off area by cutting at least a lower portion of the front surface of the lens for reducing the thickness of the lens at that portion. Though this method does not require a number of the mold assemblies to be prepared, it requires suitable means for offsetting the center axis of the front surface from the geometrical center axis of the lens to provide a desired ballast, using the precision lathe. In addition, it requires a high level of worker's skill to machine the front surface with its center axis being offset from the geometrical center axis of the lens while making good use of such offsetting means. This inevitably increases the burden of the worker, and results in an increase of the reject ratio, namely the ratio of the number of the defective products to the total number of the products manufactured.